Atkinson, F. V., On a problem of Erdős and Szekeres. Canad. Math. Bull. (1961), 7-12.

Belov, A. S. and Konyagin, S. V., An estimate for the free term of a nonnegative trigonometric polynomial with integer coefficients. Mat. Zametki (1996), 627-629. The site's key names this short note, Mat. Zametki 59 (1996), no. 4, 627-629 (Math. Notes 59 (1996), no. 4, 451-453), which bounds the least constant term of a nonnegative cosine polynomial with integer coefficients (card). The bound $\log f(n)\ll(\log n)^4$ that the site's commentary credits to [BeKo96] is in the authors' paper An estimate of the free term of a non-negative trigonometric polynomial with integer coefficients, Izv. Math. 60 (1996), no. 6, 1123-1182 (Russian original Izv. Ross. Akad. Nauk Ser. Mat. 60 (1996), no. 6, 31-90). Tang's Proc. Amer. Math. Soc. paper cites that paper for the bound, and the claim page records it.

Bourgain, J. and Chang, Mei-Chu, On a paper of Erdős and Szekeres. J. Anal. Math. (2018), 253-271.

Erdős, P. and Szekeres, G., On the product $\prod_{k=1}^n(1-z^{a_k})$. Acad. Serbe Sci. Publ. Inst. Math. (1959), 29-34.

Odlyzko, A. M., Minima of cosine sums and maxima of polynomials on the unit circle. J. London Math. Soc. (2) (1982), 412-420.

Tang, Quanyu, An improved lower bound for Erdős–Szekeres products. Proc. Amer. Math. Soc. 154 (2026), no. 8, 3381-3388, DOI 10.1090/proc/17668; arXiv:2509.14182. Not a site reference; added for the lower bound $f(n)\ge2\sqrt n$.